The present invention is related to a dynamic switch between channels and control units in a computer input/output (I/O) system, and is more particularly related to recovery of a switch connection from a possible switching error such as when the status of a connection between ports of the dynamic switch becomes uncertain.
Switches and various retry and control functions are known in the prior art. "A Communications Controller Command Set" by A. C. Rommelfanger, IBM Technical Disclosure Bulletin Vol. 14, No. 7, December 1971, pages 2233-2235, discloses making a communication controller command set independent of whether the terminal is on a switched or a non-switched line and supporting both interactive exchanges and message switching in the same network while the communications controller is shared by multiple systems. A disconnect command is used for normal termination of a session. A reset command assumes a known physical connection state.
"Use of Command Retry Function to Free Channel Interface" by M. I. Schor, IBM Technical Disclosure Bulletin Vol. 14, No. 8, January 1972, page 2284, discloses the use of a channel command retry function to enable a control unit to disconnect from the channel after receiving a command but before executing it. Described is a normal situation in which there is a known connection state but in which the control unit is not ready to proceed with the operation.
"Protocol for Wrap Operation on Duplex Loop Communication System" by R. J. Sheldon et al, IBM Technical Disclosure Bulletin Vol. 25, No. 6, November 1972, page 2840, discloses an exchange of messages which confirms that the system has been restored to a specified state. In the subject disclosure, the messages are an integral part of the process of restoration.
U.S. Pat. No. 4,648,031 issued Mar. 3, 1987 to Jenner for "Method and Apparatus for Restarting a Computing System" discloses the use of a recovery log for restarting a computer system and its interrupted units of work, and rebuilding the data base, following a normal or abnormal termination.
U.S. Pat. No. 4,665,520 issued May 12, 1987 to Strom et al for "Optimistic Recovery in a Distributed Processing System" discloses enabling a fault tolerant distributed system to recover from failures in spite of the loss of messages. The disclosed technique reduces the processing costs of maintaining the information which is essential to recovery. The status of the various processes are checkpointed and dependencies of messages on earlier messages are tracked.